While there are needs for a sealant used in solid oxide fuel cells (SOFC), it is prerequisite that such a sealant would neither degrade nor melt when exposed to high temperatures for an extended length of time, for it is a material which is to be exposed to temperatures as high as 600-800° C. for a long time. Crystallized glass has been proposed as a type of material meeting this requirement. It is also required for such a sealant to have certain flowability of the glass during the firing process, so as to provide a secure seal between various members like metals and ceramics.
At the same time, there is a problem that B2O3, a component of glass, evaporates while the sealant is kept at high temperatures and contaminates electrodes. Thus, development of a sealant is desired that does not contain evaporating components, in particular, B2O3.
Currently, many compositions for crystallized glass which have been developed so far for sealing SOFC contain B2O3 or alkali metal oxides (Patent Documents 1 and 2). While B2O3-free glass has also been developed (Patent Document 3), and SiO2—MgO-based glass is also known (Patent Document 4), they have a drawback in that they provide only insufficient crystallization. Further, those types of glass are not only apt to have higher crystallization temperatures than other glass which contain B2O3 or alkali metal oxides, but also exhibit greater viscosity during glass production, making it difficult to form them.